Today there are more people on the earth than there were in earlier centuries.
As the size of the population increases, people start to occupy or even destroy animals' habitats (the places where animals live).
Without a place to live, the animals cannot find enough food to eat or a place to raise their young.
Pollution also plays a part in extinction as more and more animals are driven from the land by people. - Where have all the Pandas Gone?, Melvin and Gilder Berger